The present invention relates to pneumatic timing valves of the kind in which one end of a plunger which projects into a chamber in constant communication with the atmosphere is displaced by fluid means acting against a return spring force, into an operating position, an incoming pressure signal initially being relieved via the chamber, and wherein a shutter element situated in the chamber and controlled with respect to time by another spring force and a restrictor system neutralizes the relief of the pressure signal after a set delay period has elapsed to generate a control signal, the restrictor system incorporating a restrictor path which is adjustable in its effective length and separately in communication with the atmosphere. Hereinafter such timing valves will be referred to as "of the kind described".
In timing valves of the kind described, the delay period set in each case is controlled by means of the atmosphere, of a groove-like longer restrictor path and a negative pressure chamber comprising a spring-loaded wall surface which carries a shutter element. Upon relieving the wall surface as a result of a displacement of the plunger by a particular distance, atmospheric air flows via the restrictor groove into the chamber, in view of the spring force acting on the wall surface and of the negative pressure thus produced in the chamber. A delay period unaffected by the operating fluid is thereby obtained. It is possible moreover to secure a comparatively long delay period by means of a correspondingly great stroke of the plunger, which may for example amount to 180 seconds.
It is a disadvantage of such a timing valve however, that delay periods which are substantially shorter than the maximum possible delay of the valve in question, can only be set in an imprecise manner. This may be attributed to the fact that in the case of shorter delay periods, the effective flow path of the restrictor groove has to be adjusted to a substantially lower value so that the negative pressure chamber may be filled rapidly with atmospheric air, to ensure that the constant stroke travel of the plunger brought into the operating position may be overbridged with corresponding speed by the following shutter element, for the purpose of terminating the delay period set. It is consequently necessary to select timing valves having a smaller maximum delay period range for cases of application involving comparatively short delay periods, which moreover establishes the need for several timing valve sizes.
It is an object of the invention to improve an atmospherically controlled timing valve having a comparatively long delay period in particular, in which the setting precision is increased considerably for delay period values below the maximum possible delay period of the valve.